The present invention is directed to an inside lining for a reaction chamber of an electric low shaft furnace, having a graphite melting crucible and a thermal insulation, and in particular to such linings wherein high purity silicon is produced by carbothermic reduction.
Typically, high purity silicon is currently produced pursuant to the Siemens C-process. According to this process, metallurgical silicon manufactured by the reduction of silicon dioxide with carbon is converted into a volatile silicon halide compound, cleaned via the vapor phase, and again reduced to silicon with hydrogen. Although silicon produced through this process meets the high purity requirements needed for such uses as electronic silicon, it is to expensive for use in many other applications such as, for example, in photovoltaics.
A possible method for manufacturing solar cell silicon suitable for photovoltaic elements in a cost-beneficial way is described in an article by J. Grabmaier in Siemens Forschungsund Entwicklungsberichten, Vol. 15, 1986, pages 157 through 162. The method is based on pre-cleaned and high purity initial materiaIs. High purity silicon dioxide is used that is obtained from glass fibers leached with hot mineral acid. High purity carbon, cleaned by leaching, is used for reduction. German published application No. 32 15 981, for example, discloses such a method.
Through the method, the pre-cleaned initial materials are reacted with one another in an arc furnace. The reaction of the pre-cleaned initial materials is based on what is referred to as the ACR process (advanced carbothermic reduction).
FIG. 1 illustrates schematically, a crossection of a known structure of an electric low shaft furnace that can be used to produce high-purity silicon. The structure essentially comprises a melting crucible 1 composed of high purity graphite or carbon, a discharge aperture 2 that is likewise lined, and a heat resistant thermal insulation 3. The thermal insulation is typically composed of refractory rock or of compounds based on silicon dioxide or, respectively aluminum oxide. A further carbon layer 4 is provided under the melting crucible for thermal insulation. A furnace jacket is provided formed by sheet steel 5.
One of the difficulties in making high-purity silicon using such a device is that it has been demonstrated that the melting crucible 1 cannot be made silicon tight. This is true even when a highly compressed graphite is used as the crucible material 1. This is due to the high seepability of the silicon melt. The silicon melt filters into the thinnest seams and enters into contact with the insulation material 3 therein. Typically, the insulation material has a high phosphorous content because binding agents containing phosphorous are used to manufacture the insulation material. When contacted with silicon, carbon, and carbonmonoxide in the furnace, the phosphorous is reduced out of the insulation material and is absorbed by the molten silicon. The silicon, which has become n-conductive due to the phosphorous content, is thereby rendered unsuitable for the manufacture of solar cells and must be cleaned of phosphorous. This is an involved process.
Problems identical to those found when phosphorous is used in the insulation material arise when almost any other element is present in the insulation material that can be reduced by the silicon melt. This is especially a problem when boron is present in that especially harmful contaminants arise from the boron. Boron, however, is contained in nearly all refractory materials. The subsequent removal of boron from the produced silicon is also extremely difficult.
Despite these problems, oxide ceramic materials for thermal insulation that exhibit an adequate purity, good insulating properties, and an adequate stability in a highly reducing furnace atmosphere are not commercially obtainable.